[autofit] Implement and use `af_get_char_index' with HarfBuzz. * src/autofit/hbshim.c (COVERAGE) [FT_CONFIG_OPTION_USE_HARFBUZZ]: Redefine to construct HarfBuzz features. (af_get_char_index) [FT_CONFIG_OPTION_USE_HARFBUZZ]: Rewritten. * src/autofit/aflatin.c (af_latin_metrics_init_blues): Use `y_offset' to adjust `best_y'.